The present invention relates to sector scan ultrasonic imaging systems, and in particular to a method and system for generating radial scan beams from the transducers of a successively selected group of a linear array as if the beams were emanating from a hypothetical point source located behind the transducer array.
A sector scan ultrasonic imaging system known in the art comprises a linear array of transducers which are successively activated by electrical pulses at times delayed among the various transducers to transmit a radial scan beam. The amount of the delay time is varied successively so that the radial beam is deflected by a predetermined increment to form a sector scan field. Since this sector field has an origin point located on the transducer array, the scanned area near the transducer array is considerably limited.
An attempt which has hitherto been made to overcome this problem involves the use of a water-filled bag between the transducer array and the surface of a body to be scanned. Although this method serves to increase the scanned area of the body, the transmitted beams are reflected at the interface between the water bag and the body. A further disadvantage of this method is that the beam tends to disperse as it propagates through the scanned area of the body with the result that the power of resolution is unsatisfactory.
Another approach which has been proposed combines the sector and linear scan methods by the use of a greater number of transducers arranged in a linear array than is employed in the sector scan type in which a group of successively arranged transducers is selected from the array and shifted by one transducer. The transducers of the groups which are located adjacent to the opposite ends of the array are activated so that they form a sector scan field near each end of the array and the transducers located between them are so activated that the beams are shifted linearly thereacross to form a linear scan field between the sector scan fields. Although this combined method is effective for expanding the scanned area near the transducer array, the scan converter which processes the returned echo signals for visual display would be considerably complicated. Another disadvantage of this method is that discontinuities tend to occur between the linear and sector scan fields.